When a photographic film is wound on a spool it is likely to take up some core-set curl, the extent of which depends on the diameter of the spool, the duration of winding (storage time) and the storage temperature. If the curl exceeds a certain prescribed limit, the film will likely have poor transport in a camera or during photofinishing operations. Because of the tendency to lower the size of film cartridges, hence decrease spool diameter, the problem of maintaining low core-set curl has become more acute. General efforts in this regard have led to the use of a high-Tg film base material, e.g., poly(ethylene naphthalate)(PEN), and costly base annealing procedures as described in U.S. Pat. Nos. 4,141,735; 5,254,445; 5,629,141 and 5,585,229. This material and other similar materials require relatively long annealing times to achieve sufficient reduction in core-set curl.
The core-set propensity of the film is often measured under extreme conditions to simulate long storage times and adverse environmental conditions—the Accelerated Core-Set Test. Such tests are conducted by winding the film tightly around an actual spool and incubating the film at high temperature for a prescribed time, usually 1 day. The wound film is then removed from the oven, allowed to equilibrate for some time at ambient conditions and its curl is measured according to Test Method A in American National Standard Institute (ANSI), P41.29-1985. The post-process curl (PPC) is evaluated by processing the film immediately after it is unwound from the spool in a standard minilab processor and measuring the residual curl of the processed film some prescribed time after the film exits the processor. Here too the curl is measured according to Test Method A in American National Standard Institute (ANSI), P41.29-1985.
Since excessive film curl can cause serious difficulties, in many imaging applications, with film transport and handling during camera use and during subsequent photofinishing, it is important to reduce the core-set propensity of the image-bearing film in accordance with system specifications.
In particular, the curl of a photographic film after photo-processing (post-process curl) must be kept sufficiently low in order to maintain acceptable performance during various photofinishing steps, e.g., printing, sleeving, autopacking, etc. In conventional 35-mm photographic films based on a cellulose tri-acetate (CTA) support, relatively high core-set curl can be tolerated. This is because a conventional CTA support possesses the capacity to reduce core-set curl during photo-processing and thereby achieve low levels of post-process curl. When using polyester film supports such as PET and PEN, post-process curl can be reduced by lowering the propensity of the film to take up core-set curl in the first place. Over the years many approaches have been taken to reduce core-set curl and/or post-process curl in photographic films. Most approaches are associated with the film base, which normally makes the most significant contribution to the core-set curl produced by the film. These approaches can be generally divided into six categories: (1) high-temperature annealing, (2) inherent curl, (3) ironing, (4) reverse winding, (5) addition of a restraining layer, (6) emulsion reformulation and (7) addition of moisture absorbing layers in a polyester laminate. Each of these approaches is applicable for certain types of films, and selection of one over the other depends on the particular circumstances of the problem at hand. Following is a brief summary of these general approaches.
(1) High temperature annealing. This method is practiced by heating the finished film usually as a wound roll to relatively high temperatures (typically 10 to 40° C. below the glass transition temperature (Tg) for relatively long times (typically>1 day) in order to lower the propensity of the film to take up curl in subsequent winding operations. This method modifies the relaxation characteristics of the film (an aged film relaxes slower than a fresh film) and is especially useful when the final winding diameter of the film is much less than the diameter during annealing. This approach is discussed in U.S. Pat. Nos. 4,141,735; 5,254,445; 5,629,141 and 5,585,229.
(2) Inherent curl. During the manufacture of film support, it is possible to induce curl in a given direction by differentially (asymmetrically) heating the film during the stretching step, i.e., by inducing a temperature gradient of ca. 10-15° C. across the thickness of the film as it is stretched above the glass transition temperature. If this inherent curl is in a direction opposite of the expected core-set curl it will compensate to some extent for the curl induced during winding and will yield lower effective curl. This method requires significant modification of the film manufacturing process and the fine-tuning of the stretching temperature of the material. This approach is considered in U.S. Pat. Nos. 4,892,689 and 4,994,214. The latter combines the inherent curl approach with physical aging; it clearly requires a fundamental change in the film-making process as well as storage for long times at relatively high temperatures.
(3) Ironing. By heating relatively short and narrow film sections to temperatures in the vicinity of Tg, it is possible to remove curl induced by core-set. This method requires some tension as the film is conveyed through the heating device and the heated film must be either flat or slightly curved in a direction opposite of the expected core-set curl. Residence times for this heating method are relatively short, of the order of minutes or less. This method is not ideally suited for treating wide and long-production rolls, however, because of the difficulty of controlling temperature uniformity and the possibility of scratching the film and damaging the coated emulsions within the ironing device. Examples of this approach are discussed in U.S. Pat. Nos. 3,916,022; 4,808,363; 4,851,174 and 5,549,864.
(4) Reverse Winding. By winding the film in the opposite direction of its induced core-set curl, the curl value can be reduced. This can be done in principle at any temperature but the rate of curl change depends on the temperature at which the film is stored and may require very long times to achieve a meaningful reduction in curl at ambient conditions. U.S. Pat. No. 3,806,574 falls under this general category, but the proposed preferred embodiment is not suitable for use in an on-line production mode, since the reverse wound roll must be stored for long times (depending on the original storage time), often greater than one day, to make an effective change in curl. In an attempt to alleviate this problem, U.S. Pat. No. 5,795,512 teaches that a combination of reverse winding and mild heating of the film can effectively reduce core-set curl after relatively short storage times.
(5) Restraining Layer. U.S. Pat. No. 6,071,682 teaches that by coating a thin polymeric layer on the side of the base opposite the emulsion, it is possible to reduce the core-set propensity of the base layer provided that the coated layer is sufficiently thick and that the glass transition temperature of the polymeric layer is equal to or greater than that of the base layer.
(6) Reformulation of the Emulsion Layer. When the base layer is relatively thin, the contribution of the emulsion layer to the overall film core-set can be significant. U.S. Pat. No. 6,485,896 teaches that formulating the emulsion with certain addenda can substantially lower the core-set propensity of the film.
(7) Addition of hydrophilic layers. A reduction in post-process curl of a polyester support can be achieved by use of a multilayered film support comprising polyesters modified by sulfonate and other hydrophilic moieties. This structure facilitates, in photo-processing, recovery of curl imposed on the film during storage in a cartridge. This approach was proposed in U.S. Pat. No. 5,556,739 to Nakanishi et al., U.S. Pat. No. 5,387,501 to Yajima et al., and U.S. Pat. No. 5,288,601 to Greener et al.
Of the above approaches, high temperature annealing is especially effective when dealing with relatively thick non-hydrophilic polyester film supports. However, this approach makes the manufacturing process more complex and less efficient because of the lengthy heating times (typically>1 day) needed to achieve acceptable reduction in core-set curl and post-process curl. Furthermore, because of the lengthy annealing times, the support is usually annealed as a wound roll. This may further prolong the annealing process because of a significant thermal lag for the wound film, and it may also produce various defects, e.g., core impressions, blocking, etc., which can lower yields and productivity.
U.S. Pat. No. 6,558,884 discloses a poly(ethylene terephthalate)-based photographic film base having improved properties with regard to cutting, perforating, and other finishing or photofinishing operations. The film base is made of a poly(ethylene terephthalate)-based material comprising a specified amount of monomeric units derived from 1,4-cyclohexanedimethanol, such that the film base has a specified cutting-related property.